I love books, during my "self taught" years of bonsai, they were my main source of information. But books have their limits, it is difficult to teach a spatial, sculptural art form, from a two dimensional media. Linear text and flat photos do not transmit the three dimensional aspect of bonsai as a sculpture related art form, and text does little to effectively relate how time is needed, and used to create these sculptural works of art. This is the reason I highly recommend, when possible, finding a teacher, or attending in person workshops, join a bonsai club, attend bonsai shows, and otherwise get hands on experience with seeing trees, and being taught how to do the techniques. Nothing beats "live, in person instruction". I understand it is not always possible, or easy, but at least occasionally get to bonsai shows, and attend at least a few workshops. If you can.
The tree species you are interested in are common enough that their horticulture is not difficult. There are tricks to learn about every species, but the general "flowering bonsai techniques" can be applied to all citrus, pomegranates, and flowering cherries, if one makes the adjustments needed for their specific needs. This is the reason that these species are a paragraph or two mention in various bonsai books, but are not the sole subject of individual bonsai books. There are very few species specific techniques for these species that are not used pretty generally across a wide number of species.
Citrus, generally are subtropical, with only a few species or varieties being tolerant of temperatures colder than a light frost. The kumquats, (Fortunella) might tolerate a little more cold in winter, but if your tree is well developed it is safer to keep it above freezing in winter rather than risk frost damage.
Poncirus trifoliata, aka Citrus trifoliata is fully winter hardy through zone 6 in the ground. In a pot it may need some protection in winter from zone 6 cold. Fruit of Poncirus is inedible, though with generous amounts of sugar, books says the juice makes a passable "lemonade", I have not tried it. My plant was a thorny beast, drew blood every time I tried to work on it, I was happy to get rid of it.
All citrus have a tendency to send branches upright, very straight, and very rigid even after only a few months. This tendency needs to be taken into account when used as bonsai. Yearly hard pruning to shorten long straight branches (as in clip and grow) or apply wire early and reapply often to get movement into the tree is crucial. Branches take several years to develop bark, they will be green until they do bark up. Scarring from wiring is possible, likely, if one leaves wire on too long. Flowers on previous summers growth. Let grow out if you want flowers and fruit.
For all the species you mention there is a gold mine of information on this website. It will take some time to dig, and read, but it is here. I know I did a multi-paragraph essay on my pomegranate several years ago. It is somewhere in the BNut threads
Sorry that books on the species you listed don't seem to exist. The information is available, just have to dig a bit.
my watcing list grows .. there is also active community of people practicing hre, other sites i checked mostly died
